subscriptions.md

Subscriptions

Subscriptions in GraphQL are asynchronous and event driven. Typically you need to have a callback installed, e.g. over a WebSocket connection, which receives the response to a subscribed query anytime it is updated. Since this library is protocol agnostic, it doesn't include the delivery mechanism. But it does provide a way to register callbacks when adding a subscription, and you can define trigger conditions when delivering an update to selectively dispatch the subscriptions to those listeners.

Adding a Listener

Subscriptions are created by calling the Request::subscribe method in GraphQLService.h:

GRAPHQLSERVICE_EXPORT [[nodiscard]] AwaitableSubscribe subscribe(RequestSubscribeParams params);

You need to fill in a RequestSubscribeParams struct with the subscription event callback, the parsed query and any other relevant operation parameters:

struct [[nodiscard]] RequestSubscribeParams
{
	// Callback which receives the event data.
	SubscriptionCallback callback;

	// Required query information.
	peg::ast query;
	std::string operationName {};
	response::Value variables { response::Type::Map };

	// Optional async execution awaitable.
	await_async launch;

	// Optional sub-class of RequestState which will be passed to each resolver and field accessor.
	std::shared_ptr<RequestState> state;

	// Optional override for the default Subscription operation object.
	std::shared_ptr<const Object> subscriptionObject {};
};

The SubscriptionCallback signature is:

// Subscription callbacks receive the response::Value representing the result of evaluating the
// SelectionSet against the payload.
using SubscriptionCallback = std::function<void(response::Value)>;

The service::await_async launch policy is described in awaitable.md. By default, the resolvers will run on the same thread synchronously.

The std::shared_ptr<RequestState> state is described in fieldparams.md.

The AwaitableSubscribe return type is a type alias in GraphQLResponse.h:

using AwaitableSubscribe = internal::Awaitable<SubscriptionKey>;

The internal::Awaitable<T> template is described in awaitable.md.

Removing a Listener

Subscriptions are removed by calling the Request::unsubscribe method in GraphQLService.h:

GRAPHQLSERVICE_EXPORT [[nodiscard]] AwaitableUnsubscribe unsubscribe(RequestUnsubscribeParams params);

You need to fill in a RequestUnsubscribeParams struct with the SubscriptionKey returned by Request::subscribe in AwaitableSubscribe:

struct [[nodiscard]] RequestUnsubscribeParams
{
	// Key returned by a previous call to subscribe.
	SubscriptionKey key;

	// Optional async execution awaitable.
	await_async launch;

	// Optional override for the default Subscription operation object.
	std::shared_ptr<const Object> subscriptionObject {};
};

The service::await_async launch policy is described in awaitable.md. By default, the resolvers will run on the same thread synchronously.

ResolverContext::NotifySubscribe and ResolverContext::NotifyUnsubscribe

If you provide a default instance of the Subscription object to the Request/ Operations constructor, or in the RequestSubscribeParams/RequestUnsubscribeParams struct, you will get additional callbacks with the ResolverContext::NotifySubscribe and ResolverContext::NotifyUnsubscribe values for the FieldParams::resolverContext member. These are passed by the subscribe and unsubscribe calls to the default subscription object, and they provide an opportunity to acquire or release resources that are required to implement the subscription.

You can provide separate implementations of the Subscription object as the default in the Operations constructor and as the payload of a specific deliver call. Whether you override the Subscription object or not, the event payload for a deliver call will be resolved with ResolverContext::Subscription.

Delivering Subscription Updates

If you pass an empty std::shared_ptr<object::Subscription> for the subscriptionObject parameter, deliver will fall back to resolving the query against the default Subscription object passed to the Request/Operations constructor. If both Subscription object parameters are empty, deliver will throw an exception:

GRAPHQLSERVICE_EXPORT AwaitableDeliver deliver(RequestDeliverParams params) const;

The Request::deliver method determines which subscriptions should receive an event based on several factors, which makes the RequestDeliverParams struct more complicated:

struct [[nodiscard]] RequestDeliverParams
{
	// Deliver to subscriptions on this field.
	std::string_view field;

	// Optional filter to control which subscriptions will receive the event. If not specified,
	// every subscription on this field will receive the event and evaluate their queries.
	RequestDeliverFilter filter;

	// Optional async execution awaitable.
	await_async launch;

	// Optional override for the default Subscription operation object.
	std::shared_ptr<Object> subscriptionObject;
};

First, the Request::deliver method selects only the subscriptions which are listening to events for this field. Then, if you specify the RequestDeliverFilter, it filters the set of subscriptions down to the ones which it matches:

// Deliver to a specific subscription key, or apply custom criteria for the field name, arguments,
// and directives in the Subscription query.
using RequestDeliverFilter = std::optional<std::variant<SubscriptionKey, SubscriptionFilter>>;

The simplest filter is the SubscriptionKey. If you specify that, and the subscription is listening to field events, Request::deliver will only deliver to that one subscription.

The SubscriptionFilter struct adds alternative filters for the field arguments and any field directives which might have been specified in the subscription query. For each one, you can either specify a callback which will test each argument or directive, or you can specify a set of required arguments or directives which must all be present:

using SubscriptionArguments = std::map<std::string_view, response::Value>;
using SubscriptionArgumentFilterCallback = std::function<bool(response::MapType::const_reference)>;
using SubscriptionDirectiveFilterCallback = std::function<bool(Directives::const_reference)>;

struct [[nodiscard]] SubscriptionFilter
{
	// Optional field argument filter, which can either be a set of required arguments, or a
	// callback which returns true if the arguments match custom criteria.
	std::optional<std::variant<SubscriptionArguments, SubscriptionArgumentFilterCallback>>
		arguments;

	// Optional field directives filter, which can either be a set of required directives and
	// arguments, or a callback which returns true if the directives match custom criteria.
	std::optional<std::variant<Directives, SubscriptionDirectiveFilterCallback>> directives;
};

The service::await_async launch policy is described in awaitable.md. By default, the resolvers will run on the same thread synchronously.

The optional std::shared_ptr<Object> subscriptionObject parameter can override the default Subscription operation object passed to the Operations constructor, or supply one if no default instance was included.

Handling Multiple Operation Types

Some service implementations (e.g. Apollo over HTTP) use a single pipe to manage subscriptions and resolve queries or mutations. You can't necessarily tell which operation type it is without parsing the query and searching for a specific operation name, so it's hard to tell whether you should call resolve or subscribe when the request is received that way. To help with that, there's a public Request::findOperationDefinition method which returns the operation type as a std::string_view along with a pointer to the AST node for the selected operation in the parsed query:

GRAPHQLSERVICE_EXPORT [[nodiscard]] std::pair<std::string_view, const peg::ast_node*>
findOperationDefinition(peg::ast& query, std::string_view operationName) const;